I have found that by sitting and preparing for the meetings together - we both understand how we wish to present our ideas and that a certain meeting plan must be adhered to, it really helps reduce him going off on tanjent. and then he knows that when I jump to the next section I am simply helping run the meeting rather than patronising him.

This may be difficult as you are dealing with your boss and it's always difficult to therefore stop yourself from overstepping the mark with regards to be in charge. But maybe some help with organisation will help keep their feet on the ground and reduce embarassment on your part, because you will know at least that they may get to the point in the end. Good luck!